How to inject the traceparent+traceid into fastapi response?

[JobaDiniz]

I'm trying to return the trace data to the client, so the client can send it back in next requests, as part of an workflow use case. There are 4 requests that should be grouped into the same trace.

1. Why isn't this done by default (or configuration) of the SDK fastapi library?
2. I'm trying to implement this, but it seems my Middleware never gets the current span - it's always Invalid_Span instance.

Middleware

class TestMiddleware(BaseHTTPMiddleware):
    def __init__(self, app: ASGIApp):
        super().__init__(app)

    async def dispatch(self, request: Request, call_next: RequestResponseEndpoint) -> Response:
        from opentelemetry import context, trace
        from opentelemetry.instrumentation.asgi import ASGIGetter, ASGISetter
        from opentelemetry.propagate import extract, inject

        # NOTE: always NonRecordingSpan(SpanContext(trace_id=0x00000000000000000000000000000000, span_id=0x0000000000000000, trace_flags=0x00, trace_state=[], is_remote=False))
        print(trace.get_current_span())
        response: Response = await call_next(request)
        # NOTE: always NonRecordingSpan(SpanContext(trace_id=0x00000000000000000000000000000000, span_id=0x0000000000000000, trace_flags=0x00, trace_state=[], is_remote=False))
        print(trace.get_current_span())

        # NOTE: trying debugging things out...
        asgi_setter = ASGISetter()
        inject(response.headers, setter=asgi_setter)
        print("INJECT: JOBAAAAAAAAAAAAAAAA")
        print(response.headers)

        return response

main

app = FastAPI()
FastAPIInstrumentor.instrument_app(app , tracer_provider=tracer_provider)
app.add_middleware(TestMiddleware)

The traces are being collected fine. I can see them in the dashboard (see screenshot), but my middleware don't the the current span, it looks like it's "closed" or something else.

And as you can see, I'm adding my middleware after the FastAPI instrumentation middleware.

[JobaDiniz]

The order matters.

So here is the implementation to inject the traceparent into the response header:

class InjectTraceParentIntoResponseMiddleware(BaseHTTPMiddleware):
    def __init__(self, app: ASGIApp):
        super().__init__(app)

    async def dispatch(self, request: Request, call_next: RequestResponseEndpoint) -> Response:
        response: Response = await call_next(request)

        scope = {}
        inject(scope)
        for key, value in scope.items():
            response.headers.append(key, value)

        return response

main.py

app = FastAPI()
app.add_middleware(InjectTraceParentIntoResponseMiddleware)
FastAPIInstrumentor.instrument_app(app, tracer_provider=tracer_provider)